1. Field of the Invention
The invention relates to ink for ink-jet printing, and more particularly, to a method of manufacturing a water base disperse ink for jet-ink recording, containing a water insoluble or slow soluble colorant, a water soluble dispersant, and a water base dispersive vehicle as essential constituents thereof
The water base disperse ink for jet-ink recording of the invention can be applied to any recording medium such as paper, cloth, and so forth, having receptivity for any water base disperse ink, and the method of 0manufacturing the same according to the invention is suited for application when manufacturing a variety of water base disperse inks containing a colorant in widely different concentration, which is required in the case of printing a pattern on a cloth, diverse in color strength.
2. Description of the Related Art
A water base disperse ink for jet-ink recording, using a water insoluble or slow soluble colorant, has the advantage over ink using a water soluble colorant in that bleeding of color is less, distinct recorded images can be formed, and images can be printed on a wide variety of recording media. This is the reason why various research and development on this subject has so far been promoted.
The water base disperse ink for jet-ink recording is generally manufactured by dispersing a pigment or a disperse dye, typical of a water insoluble or slow soluble colorant, in high concentration, into a water base dispersive vehicle (water) with the use of a dispersion apparatus such as a ball mill, sand mill, roll mill, speed line mill, homogenizer, sand grinder, or the like, diluting a dispersion liquid (dispersion liquid containing a colorant in high concentration) thus obtained with about 1 to 10 parts of a diluent, turning the same into an ink-like liquid with addition of various additives, and passing the same through a filter.
It is generally required of ink for ink-jet recording that any change in physical properties does not occur thereto during storage, clogging of ejection nozzles does not occur, and color strength and chroma of printed images are high. It is further required among other things from the viewpoint of productivity that the ink is manufactured efficiently.
It is to be pointed out that the water base disperse ink is far from satisfactory in respect of reliability since problems have often been occurred with it in particular that the dispersion liquid has undergone change in material properties over time after manufacturing owing to the inherent property thereof, that is, a colorant contained therein being water insoluble or slow soluble, causing gradual deterioration (coagulation-sedimentation) in dispersion condition of the colorant, and clogging of the ejection nozzles has resulted.
It is essential to maintain dispersion of the colorant in an even and very stable condition in order to ensure sufficient reliability.
One of conceivable measures for achieving a stable dispersion condition is to increase an amount of water soluble dispersant added so that surface activity acting between particles of the colorant and the water base dispersive vehicle by the agency of the water soluble dispersant can be further enhanced.
However, from the viewpoint of securing ejection characteristics of the nozzles, it is necessary to keep the viscosity of the ink at a given level or lower, and this requires an amount of the water soluble dispersant added to be controlled at the lowest possible level.
The water soluble dispersant has thus been faced with needs for satisfying contradictory requirements, and it has been extremely difficult to meet the both requirements at the same time.
Meanwhile, a method of manufacturing a pigment ink having a steep grain size distribution curve whereby a dispersion liquid after milling is diluted by adding various additives thereto, and fine-dispersion is again applied thereto by use of a high pressure homogenizer, as disclosed in Japanese Patent Laid-open Publication No. H 8-209045, has been proposed as means for achieving an even and stable dispersion condition represented by a steep colorant grain size distribution curve without containing coarse colorant particles.
The manufacturing method described is characterized by the use of a high-pressure homogenizer whereby a dispersive vehicle containing a colorant, disperant, and the like is caused to pass through micro-orifices at a high speed under high pressure in order to achieve even dispersion, and consequently, a stable water base dispersion liquid having a steep grain size distribution curve is obtained because the larger the mass of a particle, the faster the particle is accelerated. However, this method is still less than satisfactory from the viewpoint of securing absolute reliability on a long term basis.
Clogging of the nozzles occurring with the elapse of many hours in use is attributable to the fact that particles of the water insoluble or slow soluble colorant are not dispersed in stable condition.
Further, in the case of diluting a dispersion liquid with a colorant in high concentration to manufacture a water base disperse ink with emphasis on enhanced productivity, if an attempt is made to improve productivity by adding a diluent (water base dispersive vehicle) and various additives for adjusting ink characteristics to the dispersion liquid with the colorant in high concentration, and diluting the same at a high dilution factor, the consistency of liquid will undergo a significant change upon dilution, and dilution coagulation (the so-called solvent shock) will occur, causing a problem that a water base disperse ink having a stable dispersivity can not be obtained due to deterioration in dispersivity.
A dilution factor at which the diluent is added to, and mixed with the dispersion liquid containing the colorant in high concentration to obtain a disperse ink is normally regarded to be on the order of 1 to 10 parts of the diluent at one dilution, and if production efficiency is to be enhanced by raising the dilution factor to 10 parts of the diluent or higher while holding back deterioration in dispersivity, dilution operation needs to be repeated several times.
Time required for executing such a process and complexity of the process for repeating the dilution operation constitute a major negative factor for improving production efficiency.
Although it seems possible to improve productivity by setting concentration of the colorant and dispersant, contained in the dispersion liquid with the colorant in high concentration, at higher levels, it then becomes necessary to raise the dilution factor, leading to eventuality of the dispersivity becoming lower due to the solvent shock. For avoiding this, the number of process steps for repeating dilution operation needs to be increased, and consequently, the proposed method does not contributes to enhancement in production efficiency.